This invention relates to a rotation sensor of a rotating body such as a wheel of a vehicle, and specifically to an abnormality detecting device for detecting a breakage of wiring of the rotation sensor.
When a vehicle running at high speed undergoes a strong emergency braking, the wheels are locked and a skid occurs between the wheels and the road surface because there is a limit in the friction between the tire and the road surface. In order to prevent the skid, an anti-skid control is already known in which the braking force is controlled to keep the slippage (slip ratio) between the tire and the road surface at a predetermined adequate value.
During anti-skid control, rotation speed and rotation acceleration of the wheel must be accurately monitored, and for detecting the rotation speed, a rotation sensor is provided for the wheel. The rotation sensor includes a hub rotor and an electromagnetic pick-up. The hub rotor rotates with the wheel and has many projections arranged on its periphery. The pick-up, on the other hand, is fixed to the vehicle body in close proximity to the hub rotor and senses an access of each of the projections with a coil and a permanent magnet. With each access of the projection to the coil due to the rotation of the wheel, the magnetic field around the coil is altered to generate a voltage signal at the coil. By counting the number of voltage signals generated with respect to time, the rotation speed of the wheel is detected.
One of the possible abnormalities of such a rotation sensor is a breakage of the wiring of the coil or its neighbors. For detecting such breakage, a method is already known, in which a detection signal of a preset voltage height is sent to the rotation sensor and a change in the detection signal is checked when the ignition key switch is turned on at the time of engine start-up.
However, abnormality of the rotation sensor in the course of the anti-skid control is not detected by checking the breakage only at the time of engine start-up. Thus, if the breakage occurs when the vehicle is running, the rotation speed of the wheel is sensed as 0 because the rotation sensor will not generate the voltage signal. This leads to an incorrect judgment that the wheel is locked and causes a release of the brake by the anti-skid control system, which contravenes the intention of the driver.